Carried or caused by something in the soil, like a disease or a plant's roots.
Soilborne things can be bad because they can make plants sick, but they can also be good because they help plants grow.
Examples:
The soilborne disease made all the plants sick.
The soilborne roots spread under the ground.
We need to stop the soilborne problem before it gets worse.
The soilborne plants are taking over the garden!
The soilborne magic made the flowers grow really big.
The soilborne problem is getting bigger and bigger.